The development of the pattern of peroxidatic activity (PA) has been studied in vivo and in vitro in macrophages and other mononuclear phagocytes. In vivo in the animal model the macrophages show four different PA patterns characterizing exudate macrophages, exudate-resident macrophages, resident macrophages and PA-negative macrophages. Cultured in vitro, rat and human blood monocytes and rat, mouse and human exudate macrophages acquire the characteristic PA pattern of resident macrophages via the transitional stage of cells with the characteristics of exudate-resident macrophages. The cytochemistry, the occurrence of this cell type in vivo and the kinetics in vitro indicate that exudate-resident macrophages represent a transitional form between the exudate and the resident macrophages. The results obtained in vivo and in vitro strongly suggest that divergent PA patterns of mononuclear phagocytes represent differences in the stages of development of these cells in the sequence monoblast - promonocyte - monocyte - exudate macrophage - exudate - resident macrophage - resident macrophage. Moreover, PA-negative cells may in fact represent, firstly, another transitional stage between exudate and resident cells and, secondly, an end-stage macrophage in vitro.